Corrugated board is a paper sandwich construction made by combining three or more paperboard sheets. In its most common form, two paperboard facings are adhered to each side of an inner paperboard sheet which has been formed into a sinusoidal shape. This construction achieves high stiffness and strength with low weight and cost.
Shear stiffness is an important property for corrugated board. If a corrugated board is well specified with strong liners and medium, it will perform as designed, but if the medium is damaged during or after manufacture its strength may be sub-standard.
Flat crush of corrugated board is often used to measure the “strength” of the corrugated board. The flat crush test is performed on a circular sample of a standard area. The specifications for this test are given in Australian Standard 1301.429s-89. A disadvantage of this test method is that it will not discriminate between good undamaged board and corrugated board damaged during processing that feels soft to the touch. This is because the highest crush result occurs after the flutes have been well squashed. Damage to the core material by excessive lateral pressure can severely weaken the core structure. A crushed core exhibits a drastic loss of machine direction global shear stiffness of the core and will also lack hardness and bending stiffness.
Shear stiffness is a fundamental structural property of a sandwich panel and is defined by:
                    Shear_Stiffness        =                  Shear_Force          Shear_Strain                                    Eq        .                                  ⁢        1            
Shear stiffness in the machine direction is designated as Gxz and in the cross direction global as Gyz.
U.S. Pat. No. 3,955,409 discloses a device for torsional oscillation testing of plastics materials.
U.S. Pat. No. 4,958,522 describes a method for determining the shear stiffness of corrugated board by mounting and twisting a test piece between two clamps. Measurements are taken of the angle of twist and the force applied using a rotary encoder and a load cell. In use of this technique for quality assessment of corrugated board it is assumed that with all else being equal (liner compression etc) damage during conversion is the main cause of poor corrugated board performance and the shear test will show up this damage.